Are Venus' mystery clouds warning sign for Earth?

Scientists say they may be a cautionary tale about climate change

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This false-color ultraviolet image of the south pole of Venus was obtained by the Venus Monitoring Camera onboard The European Space Agency's Venus Express on Feb. 25, 2008 from a distance of about 12,400 miles. The octagonal shape of the image is due to the VMC field of view.

Mysterious clouds discovered on Venus are not only helping scientists learn more about Earth's closest planetary neighbor, they also serve as a cautionary tale for one proposed method of combating climate change on our own planet.

The peculiar sulfur dioxide clouds of Venus have perplexed scientists since they were first discovered in 2008 by the European Space Agency's Venus Express spacecraft.

Now, computer simulations made by Xi Zhang, a graduate student in the Division of Geological and Planetary Sciences at the California Institute of Technology in Pasadena, Calif., may explain these high-altitude clouds. [Photos: Beneath the Clouds of Venus]

The study may also mean that one idea for addressing global warming on Earth — injecting sulfur droplets into Earth's atmosphere — may not be as effective as previously thought, researchers said.

Sulfuric acid in Venus air
Zhang and colleagues from the U.S., France and Taiwan show that some sulfuric acid droplets may evaporate at high altitudes, freeing gaseous sulfuric acid that is then broken apart by sunlight, which in turn releases sulfur dioxide gas.

Venus is blanketed in clouds of sulfuric acid that obscure our view of the planet's surface. These clouds typically form at altitudes of about 30 to 40 miles, when sulfur dioxide from volcanic activity combines with water vapor to form droplets of sulfuric acid.

At elevations higher than approximately 43 miles, any remaining sulfur dioxide should be rapidly destroyed by exposure to intense solar radiation at such altitudes. So, when Venus Express detected a layer of sulfur dioxide at altitudes of about 56 to 68 miles, it left scientists scratching their heads.

"We had not expected the high-altitude sulfur layer, but now we can explain our measurements," said Håkan Svedhem, ESA project scientist for Venus Express. "However, the new findings also mean that the atmospheric sulfur cycle is more complicated than we thought."

Warning sign for Earth climate fixes?
While the findings shed light on the nature of Venus' hellish atmosphere, they may also disprove the effectiveness of one suggested technique for mitigating climate change on Earth.

The proposal stemmed from observations of the aftermath from powerful volcanic eruptions. In particular, the 1991 eruption of Mount Pinatubo in the Philippines shot deposits of sulfur dioxide into Earth's atmosphere. Reaching an altitude of 12 miles, the gas formed small droplets of concentrated sulfuric acid, similar to those found in the clouds on Venus.

These clouds then spread around Earth, and the droplets created a layer of haze that reflected some of the sun's rays back into space, which cooled the whole planet by approximately 0.9 degrees Fahrenheit (0.5 degrees Celsius).

In light of these new findings, injecting sulfur droplets into Earth's atmosphere may not be as effective a method of battling climate change as once thought. For one, it is not known how quickly the initially protective haze will be converted back into gaseous sulfuric acid, which is transparent and allows the sun's rays to pass through.

"We must study in great detail the potential consequences of such an artificial sulfur layer in the atmosphere of Earth," said Jean-Loup Bertaux, a researcher at the Universite de Versailles-Saint-Quentin in France. Bertaux is also the principal investigator for the SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) sensor on Venus Express, an instrument that probes the Venusian atmosphere.

"Venus has an enormous layer of such droplets, so anything that we learn about those clouds is likely to be relevant to any geo-engineering of our own planet," Bertaux said.